In general, encoding a moving image (video) signal for transmission in a mobile communication system is based on a H.263 video coding standard. A most commonly used method for controlling an encoding bit rate of the H.263 video encoding is the method proposed in TMN 5 (Test Model Near-term 5).
Although performing well with respect to the encoding bit rate, the method proposed in TMN 5 is disadvantageous with respect to maintaining the best image quality, specifically, minimizing distortion. This is because the selection of the macroblock encoding mode, such as INTRA, INTER, SKIP and the like, is determined based on a generated bit amount (i.e., coder output buffer occupancy) and not on distortion aspects.
As a way of compensating for the deficiencies in the TMN 5 method, a most representative and optimized method for controlling a bit rate in the related art R-D (Rate-Distortion) sense has been proposed by Thomas Wiegand et al. in “Rate-Distortion Optimized Mode Selection For Very Low Bit Rate Video Coding And The Emerging H.263 Standard”, IEEE Transactions On Circuits And Systems For Video Technology, Vol. 6, No. 2, April, 1996. The aforementioned technique uses a coding control parameter (for selecting the encoding mode) determined in accordance with an R-D relationship, namely, the Lagrange multiplier (λ), not the quantization parameter.
FIG. 1 is a graph illustrating a general R-D relationship. As shown, if the target bit rate (Rt) is determined in the R-D relationship, which is obtained by characteristics of a frame, the Lagrange multiplier (λ) is determined. In contrast, if the Lagrange multiplier (λ) that corresponds to the negative slope of the function (curve) in the R-D graph is determined, then the corresponding bit rate and distortion are accordingly determined. This means that the bit rate may be controlled by using the Lagrange multiplier (λ).
Wiegand et al. proposes that the value of λ may be dynamically obtained for each frame or each GOB (indexed by K) of a video sequence by the following equation (1).
                              λ                      k            +            1                          =                              λ            k                    +                                    (                                                                    R                    c                                                        R                    k                                                  -                1                            )                        .                                              equation        ⁢                                  ⁢                  (          1          )                    
In equation 1, k represents the order of the kth frame or GOB, and λk+1 represents the value of λ for the k+1th frame or GOB. Also, Rk represents the encoding bit rate generated by encoding the kth frame. Rc represents the rate constraint, being the target bit rate required for the encoded frame or GOB.
In equation (1), the value of λ for the current frame is determined by the encoding bit rate (Rk) generated by encoding the previous frame and the target bit rate (Rt) required for the encoded previous frame.
FIG. 2 is a diagram illustrating the related art principle of an inter-frame λ value update. As shown, the value of λ is updated by an inter-frame update method in the related art. Namely, the encoded bit rate of a first frame is controlled on the basis of a value λ1, and the encoded bit rate of a second frame is controlled on the basis of a value λ2, which is obtained based on the value of λ1. Accordingly, the encoding bit rate of the kth frame is controlled on the basis of a value λk, which is obtained based on the value of λk+1.
As mentioned above, the related art inter-frame coding parameter update method in which the value of λ for the current frame is obtained based on the value of λ used for a previous frame is advantageous in encoding an image signal with a low bit rate. Here, the bit rate is low when the characteristics of the image signal of the previous frame are almost the same as those of the current frame and there is little motion.
However, in the related art inter-frame coding parameter update method, if the bit rate to be used by units of a frame increases due to higher motion, an error in the generated bit rate undesirably increases. Thus, the bit rate cannot be effectively controlled thereby causing an image received by a receiver to be greatly changed in quality.
Specifically, the method in which the value of λ is updated by units of a frame to control the target bit rate has the following problems. First, since the fluctuation in generated bit rates between frames can be great, the image quality can be very different frame by frame. Also, a decoder side requires a bigger-sized buffer. Furthermore, an image signal with high quality cannot be transmitted to a receiver.